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How does sideslip indicator react during crosswind?
I will basically consider two types of sideslip indicator: the yaw string and the marble/ball inside a curved tube. I don't know the exact aeronautical term for both and I don't know if other sideslip indicator exists.
The pilot should maintain no sideslip, i.e. keeping the string centered. Let's say the pilot is able to do it. What happen in case of cross wind? Considering the air mass as frame of reference, the aircraft fly straight forward, but considering the earth as frame of reference, the aircraft is flying a bit sideways (crabbing). Thus the yaw string and the marble/ball may disagree. Am I right?
Going further, if the aircraft fly straight forward in an airmass (no sideslipping) with winds and gusts, the aircraft will experience sideways accelerations. How will the marble react? Will its indication still be reliable?
EDIT: Given comments and answers, I need to provide an extreme situation: the constantly increasing crosswind. The air mass is no longer an inertial frame of reference. Keeping no side slip result in an increasing crabbing. My understanding is that the ball should react to this increasing crabbing (sideway acceleration relative to an inertial frame of reference) but not the string (no side slip relative to the air mass)

Imagine you are flying in a cloud, making coordinated turns with respect to your instruments, and cannot see the ground.  Can you tell what direction the airmass you are flying in is going with respect to the ground?  No, you only know that your flight is coordinated (i.e. string centered).

@CrossRoads totaly agree, that's why I'm asking how would the ball/marble reacts. This indicator can only react to accelerations in an Inertial frame of reference (such as the ground) while the string give an indication relative to the airmass

If you are in coordinated flight, the ball remains centered. If you can see the ground and overcorrect (because for instance, you are turning base to final and the wind is blowing you past the runway centerline), that's when folks get into trouble, and stall too low to recover and put the plane into the ground.

@CrossRoads I think we misunderstand each other. my question is: if you are not sidesliping (turning or not, it doesn't matter), keeping the string centered, but the airmass is not still, will the ball stay centered?

Yes; you will just be pushed off your intended flight path. That's why look up the winds ahead of time and plan direction corrections in.  If you are flying west, and the wind is coming from the south, you get pushed off your route to the north. So you do some math and figure out how much left of the heading you want to fly to compensate for the wind. For example, you might fly a heading of 255 to keep your ground track going in the 270 direction and along the ground path you wanted.  Ball/string will still indicate coordinated flight; from the ground you will appear to be going sideways.

@CrossRoads what confuses me is that if you are shaking your aircraft, the ball won't stay centered, but if the wind is shaking the aircraft and you can keep the string centered, the ball has no reason to know if you are in coordinated flight.

But shaking is normally felt vertically from rough winds, not horizontally.

considering the earth as frame of reference, the aircraft is flying a bit sideways (side slipping?). This is not sideslipping, this is crabbing. Sideslip has nothing to do with the ground at all, only the air mass.

@TomMcW edited accordingly. (thanks for the vocabulary)

@ManuH Situation on ground is very different from in flight. These should be two different questions. I don't know what further can be explained by existing answers.

@quietflyer should I only keep the part about the constantly accelerating crosswind?

I think the latest edit (edit 9) improved the question by removing the roller coaster/ train content, thanks.

@quietflyer I'm putting it into a new question dedicated to the ball

I fail to understand how the instrument fixed to aircraft can move (and thus accelerate longitudinally) relative to the aircraft (e.i. in the aircraft's reference frame)

Let us continue this discussion in chat.

We should start by understanding what the deflection of the "inclinometer" ball actually indicates.  To a good approximation, it responds to the net sum of all lateral (sideways) accelerations acting on an aircraft, excluding the lateral acceleration component due to gravity.  By "lateral" or "sideways" we mean in the aircraft's own reference frame -- for example, in a turn, the direction meant by "lateral" or "sideways" would be constantly changing as seen by an observer on the ground, or for that matter, an observer in a balloon drifting by with the airmass.  In a normal coordinated turn, the net lateral acceleration other than that due to gravity is zero.  To a first approximation, the only thing that causes an aircraft to experience a lateral acceleration, other than gravity, is the aerodynamic sideforce generated by a sideslip as the airflow strikes the side of the fuselage.  That is why the ball serves as a slip-skid indicator, i.e. a sideslip indicator.  There are some reasons why the ball is not a perfect indicator of sideslip, which will become evident later in this answer, but the idea that the ball is basically a sideslip indicator is a good starting point.
Although it's not germane to this answer, to be complete we should also point out that for a given aerodynamic sideforce, the motion of the ball is also influenced by the G-loading, or more precisely, by the upward aerodynamic force generated by the wing.  The higher the "upward" G-loading, the more aerodynamic sideforce it takes to cause a given deflection of the ball.  At zero "upward" G's, the slightest aerodynamic sideforce will put the ball all the way into one of the far corners of the glass tube, and at negative "upward" G's, the ball tends to just stay stuck in one of the corners even when sideforce is zero.  This is why many aerobatic aircraft have a second slip-skid ball that is mounted upside-down.
If the ball is an accelerometer, how can it be deflected in a situation like a steady-state wing-down slip along a straight-line flight path, where net acceleration is clearly zero?  The answer is that net acceleration is zero, but there is still an aerodynamic force component, and therefore an aerodynamically-driven acceleration component, acting in the sideways direction in the aircraft's reference frame, caused by the air impacting the side of the fuselage.  Just as a panel-mounted G-meter reads "1" not "0" in straight-and-level flight-- net acceleration is zero, but there is still an aerodynamic force component acting in the upwards direction in the aircraft's reference frame-- the lift force from the wing.  Just as the slip-skid ball measures the net lateral (sideways) acceleration other than that caused by gravity, so too does the G-meter measure the net vertical acceleration component other than that caused by gravity, which is essentially just the upward acceleration caused by the wing's lift vector.
What reference frame is the slip-skid ball operating in?  We've seen that the slip-skid ball measures lateral acceleration (other than that due to gravity).  We can measure net acceleration in any valid inertial reference frame and we'll get the same answer, except for differences due to the tilt of one frame relative to the other.  An accelerating airmass is not a valid inertial reference frame.  Nor is the aircraft itself, unless net acceleration is zero.  Therefore it is not wrong to assert that in the context of an accelerating airmass, the slip-skid ball is operating in the reference frame of the earth, rather than the reference frame of the aircraft or the airmass, except that the reference frame is tilted to match the orientation of the aircraft in space at any given instant in time, and so is constantly changing its orientation with respect to the orientation of the earth's reference frame.  In other words the direction called "lateral" is fixed with respect to the aircraft, not the earth-- it changes (relative to the earth) as the aircraft changes heading, or as the aircraft banks.  Yet the actual acceleration in that (ever-changing) lateral direction is measured in relation to a valid inertial reference frame, not in relation to the aircraft itself.  It is a valid question to ask how it can be that the slip-skid ball which in a sense is tied to the earth reference frame, can give essentially the same indication as the yaw string.  Perhaps the above paragraphs have shed some light on those questions-- the answer has to do with the fact that the slip-skid ball is only measuring the lateral component of acceleration, and the fact that the lateral component of acceleration is intimately related to sideslip.  Keep in mind that in many instances in this answer, we use phrases like "in the aircraft's own reference frame", but we really only mean to describe the orientation of the reference frame involved.  At any instant time, the actual reference frame that an on-board accelerometer such as the G-meter or the slip-skid ball is operating in would indeed be an actual valid inertial reference frame such as that of the earth itself, just tilted differently.  It is correct to observe that an accelerometer is not actually operating in the reference frame of the aircraft itself in any case where the aircraft is accelerating, or in the reference frame of the airmass in any case where the wind is not steady.  If the slip-skid ball were actually operating in the reference frame of the aircraft itself, it would always read zero, and likewise the G-meter.
Your first specific question is essentially "if the pilot is keeping the yaw string centered, will the presence of a steady crosswind cause the ball to deflect?"  Absolutely not.  The plane flies within the airmass, or "inside of the wind".  It doesn't "feel" a steady wind in any way.  Like if you were flying a tiny model airplane inside a car driving steadily down the highway at 100 mph-- the plane doesn't know which way the car is driving.  The pilot doesn't have to do anything different with the rudder to keep the yaw string (or ball) centered when there is a steady crosswind, than when there is not.
You'll probably have an easier time understanding the effects of crosswind gusts if you assume that the pilot is not using the rudder pedals to try to keep the yaw string perfectly centered.  The gust will deflect the yaw string downwind and the ball upwind until the aircraft has accelerated sideways sufficiently to be back in equilibrium with the new state of movement of the airmass.  During that time of acceleration, while the airplane is "feeling" the sideways wind component, the vertical fin is also exerting a yaw torque to yaw the nose into the gust.  How much change in heading happens before the aircraft reaches equilibrium with the new state of the airmass will depend on how big the fin is and how much rotational inertia the aircraft has about the yaw axis.  This will vary from one aircraft to another.  Of course, the change in heading causes the acceleration from the gust to become less of a lateral acceleration and more of a longitudinal acceleration, in the aircraft's reference frame.
Lest anyone get the wrong idea, we should note that a pilot usually does not apply rudder to correct for a crosswind gust, except perhaps during final approach when the wheels are about to touch the ground and small deviations in the aircraft heading cannot be tolerated-- and then the correction is in the opposite direction that we are talking about here.  As far as keeping the nose pointing into the relative wind, normally the vertical tail takes care of things well enough on its own, by creating a rudder-like effect that yaws the aircraft.  This is sometimes called the "weathervane effect", although it is important to understand to understand that it is driven only by the "relative wind" that is felt by the aircraft, and is not influenced in any way be a steady wind.  Since this yawing does not happen instantly, the side of the aircraft is temporarily exposed to the sideways airflow component created by the gust, so we'll see a temporary deflection of the ball in the upwind direction and the yaw string in the downwind direction.
A recent addition to the question has raised the question of what behaviour we would see in an aircraft with essentially zero mass.  How will such an aircraft respond in the presence of a sideways gust, or a steady increase in horizontal wind speed?  The aircraft will accelerate instantly with the gust or the increase in wind speed and there will be no tendency at all for the vertical fin to generate any yaw torque at all.  The aircraft heading will have no tendency to change.  Yet the yaw string will stay centered.  Because there is lateral acceleration, the slip-skid ball will indeed not stay centered -- assuming that the slip-skid ball somehow does exist in such an aircraft.  What we've done here is by specifying zero mass, we've broken the intimate connection between sideslip and lateral acceleration.  With an infinitely small amount of mass, an infinitely small amount of sideslip is needed to drive a given lateral acceleration.  In the real world, we will indeed see less deflection of the slip-skid ball for a given deflection of the yaw string when a gust strikes a heavily loaded aircraft, than when it strikes an aircraft of identical shape and size but less mass. A much simpler analogy would be two toy cars of the same shape and size but different mass, each with a flag on it.  When a wind gust strikes both cars and blows both flags in an identical manner, the wind will accelerate the lighter car faster than the heavier car.  The flags are analogous to the yaw string, and the cars are analogous to the aircraft accelerating sideways and tending to "leave the ball behind", so that the ball deflects sideways inside its tube.
Now for the part of your question about the superman pilot with lightning-fast reflexes making control inputs as needed-- if needed-- to keep the yaw string perfectly centered as a wind gust strikes.  In a real aircraft, with non-zero mass and a non-zero moment of rotational inertia about the yaw axis, some control inputs will definitely be needed -- in essence, the pilot will be doing some fancy footwork on the rudder pedals to "help" the vertical fin do a better job of keeping the nose of the plane pointing directly into the "relative wind" so that the yaw string stays absolutely perfectly centered even as a strong gust strikes from the side.  Some aileron input may be needed as well to keep the wings level-- surely we don't want to complicate the problem by allowing the plane to bank as the gust strikes.
Let's start by simplifying the problem by assuming that the rudder pedals are connected to wingtip drag-producing devices that, unlike a conventional rudder, generate no net sideforce when they are deployed into the airflow.  They only generate yaw torque.
Basically we're saying that the pilot is using the rudder pedals to "help" the fin "weathervane" the nose to point directly into the relative wind, in such a perfect manner that the plane never experiences any sideslip at all even as a gust strikes.
This means that the aircraft will never feel lateral accelerations, only longitudinal accelerations.  As the gust strikes, the aircraft will tend to pitch up and climb, but to a first approximation the ball will remain centered along with the yaw string.
Why only "to a first approximation"?  Well, the ball is not quite perfect as a sideways accelerometer.  The rate of yaw rotation about the aircraft's CG can also influence the ball.  Imagine that we mounted the aircraft on a pivot at its CG in a vacuum-filled hangar and twirled the airplane like a pinwheel in the yaw dimension.  The ball would deflect opposite the direction that the nose was moving.  The further the cockpit from the CG, the more pronounced this effect would be.
Normally this effect is negligible in actual flight.  How important it is in your thought experiment depends on how "sharp-edged" the gust is-- how quickly it ramps up.  Does it take two seconds to reach full strength?  Half a second?  Zero time at all?  If the latter, the aircraft must yaw at an infinite rate of speed to stay aligned with the relative wind and keep the yaw string centered, and the ball will be ejected out the downwind side of the tube!
In a later edit you suggested that you were interested in the case where the wind just keeps getting stronger and stronger indefinitely.  Let's continue with our "fancy footwork" thought experiment where the pilot "helps" the fin to eliminate all sideslip so that the yaw string stays centered, and (to a first approximation) the ball stays centered too.  Again, in the aircraft's own reference frame, no lateral (sideways) acceleration is taking place, only longitudinal acceleration.  If the wind is initially zero, and starts blowing directly cross to the aircraft's original course, then as the wind gets stronger and stronger, before long the aircraft will be pointing 45 degrees off its original heading, and then before much longer the aircraft will be pointing essentially directly into the wind, and there will be no need for any more rudder pedal inputs to keep the yaw string and ball centered, even as the wind speed continues to increase.  Of course, the aircraft's groundspeed will soon be negative, and getting more and more so.  While the aircraft will have maintained its original groundspeed component in the direction that is cross to the wind, this will soon have a negligble effect on the direction of the ground track because the groundspeed component in the downwind direction will be so large.  If the elevator and throttle are left in the same position that originally yielded level flight, the airspeed will stabilize at a value that is close to its original value, but the aircraft will be climbing, due to the extra energy constantly added into the system by the constant increase in wind speed.  The situation has parallels to the "dynamic soaring" method used by the albatross to exploit the wind gradient over the open ocean.
At this point you may feel the plot has gotten convoluted enough and the story should end.  And it probably should.  If you are still having any trouble understanding any of the above content, please stop reading now and review!  Especially if you still are not convinced that an aircraft does not "feel" the presence of a steady wind.  Your time and effort are better spent trying to understand that concept, then in diving into any of the micro-level details that follow.
But if you feel ready for yet a few more twists and turns--
What happens if the pilot uses a rudder to yaw the aircraft?  A rudder generates its own aerodynamic sideforce, even when the rest of the aircraft does not.  When we step on the left rudder pedal, we deflect the rudder to the left, creating an aerodynamic sideforce to the right as well as a yaw torque to the left.  Often this yaw torque changes the orientation of the aircraft relative to the flight path, so that the right side of the aircraft is exposed to the airflow, which creates an aerodynamic sideforce to the left that dwarfs the opposing sideforce from the rudder itself.  (See for example this related ASE answer.)  So the ball is deflected to the right-- opposite the direction that the yaw string is deflected.  But in the case of our thought experiment with the wind gust and the "perfect" correction by the pilot, the aircraft stays perfectly streamlined to the airflow, and nothing opposes the rightwards aerodynamic sideforce from the deflected rudder, so the ball is deflected to the left-- in the same direction that the pilot deflected the rudder.  This comes into play with dealing with a failed engine in a twin-engined aircraft-- see footnote 1 for more.
Again, how important this effect is in our thought experiment depends on how "sharp-edged" the gust is-- how quickly it ramps up.  Does it take two seconds to reach full strength?  Half a second?  Zero time at all?  If the latter, the aircraft must yaw at an infinite rate of speed to stay aligned with the relative wind and keep the yaw string centered, and the sideforce from the deflected rudder would tend to cause the ball to be ejected out the "upwind" side of the tube!
And now the million-dollar question-- in a gust that ramps up at a non-infinite rate, which of the two above effects dominates?  The centrifugal force from the rotation of the aircraft about its own CG, which tends to move the ball in the "downwind" direction, or the sideforce from the deflected rudder, which tends to move the ball in the "upwind" direction?  Almost certainly the latter, for most real aircraft.  But let's consider the extreme cases--
Extreme case 1-- the rudder is very far aft of the CG, and the aircraft has a very low moment of rotational inertia about the yaw axis, and very little aerodynamic damping in yaw (e.g. a small vertical fin) so that only a very little sideforce from the rudder is needed to establish a given rate of rotational acceleration in the yaw axis, or to maintain a given rate of yaw rotation.  The cockpit is very far in front of the CG.  The first effect-- the "centrifugal force" effect-- will dominate and the ball will move in the opposite direction as the rudder input.
Extreme case 2-- The rudder is very large but is only a very short distance behind the CG, and the aircraft has a very high moment of rotational inertia about the yaw axis, and lots of yaw damping (e.g. lots of side area behind the CG), so that lots of sideforce from the rudder is needed to establish a given rate of rotational acceleration about the yaw axis, or to maintain a given rate of yaw rotation.  The cockpit is only a short distance in front of the CG.  The second effect-- the sideforce from the rudder-- will dominate and the ball will move in the same direction as the rudder input.
Now we've gone and opened another can of worms here as well-- the difference between the rudder input required to establish a given rate of rotational acceleration about the yaw axis, versus the rudder input required to maintain a given rate of rotation about the yaw axis.  What is the appropriate response to a gust that ramps up linearly? Or does the gust perhaps increase in a sinusoidal manner rather than a linear one?
Note that to keep a steady yaw rotation rate going once established, we'll still need to keep the rudder deflected.  This is due to "yaw damping"-- if the nose of the aircraft (where the yaw string is located) is perfectly streamlined to the airflow, then the further-aft parts of the aircraft cannot be, due to the differences in the velocity of the local relative wind induced by the yaw rotation itself.  (E.g., points on the tips of different blades of a pinwheel or windmill are moving through the airmass in different directions at any instant in time.)  And where is the torque that creates this yaw damping coming from?  Actually, in this case where the nose of the aircraft is specified to be streamlined to the flow, it's coming largely from the rear parts of the aircraft being pushed sideways through the air.  So we do have some aerodynamic sideforce opposing the sideforce from the rudder, after all.  If the rudder is deflected and the yaw rotation rate is constant and there is no opposing yaw torque (like that due to a failed engine-- we're assuming the increased drag from the faster-moving outboard wingtip is negligible in the grand scheme of things), then we have to conclude that the net sideforce is in fact in the same direction as the rudder is deflected, after all.  So in this case, after our initial yaw rotation rate is established, the ball will be deflected in the opposite direction as the rudder is deflected.  Both the "centrifugal" effect from the yaw rotation, and the effect from sideforce, are working together.  (And to be complete, we need to point out that this sideforce from yaw damping also exists in the case of the aircraft that is yawed with tip-dragger devices.)  But when we were first establishing the yaw rotation rate, the ball may have been doing something different.
Don't spend any time trying to understand these nuances until you are first completely solid on the idea that an aircraft does not feel the presence of a steady wind in any way.  In aviation, that's kindergarten stuff.  To complete the analogy, getting into the nuances of differences between the behavior of the yaw string and the ball in various situations involving rudder deflections and yaw rotations is more of a graduate-school-level topic.  Understanding that the ball responds mainly to aerodynamic sideforce, and the rudder creates some sideforce whenever it is deflected even if the side of the fuselage is not exposed to the airflow-- that falls somewhere in between (high-school level?), and pilots routinely put that into practice when they leave the ball deflected toward the working engine when practicing for engine failure, or dealing with actual engine failure in a twin-engined airplane.
Postscript 1-- I suspect that what you really want to ask, is "what happens if the aircraft magically rotates as needed to stay perfectly aligned with the relative wind as the crosswind gust ramps up, while also bending like a banana so that the yaw rotation itself creates no variations in the direction of the local relative wind experienced by various parts of the aircraft."  In this case the ball moves in the "downwind" direction to some degree that is determined by the aircraft's yaw rotation rate, due to the centrifugal force effect, but the aircraft experiences no lateral acceleration, only longitudinal acceleration.
Footnote 1-- This is like what happens when we use the rudder to control a twin-engine aircraft with one failed engine-- when the fuselage is streamlined to the airflow, the ball is slightly deflected in the direction of the deflected rudder, so we should refrain from applying as much rudder as would be needed to fully center the ball and bring the turn rate to zero with the wings level.  Instead we should leave the ball slightly deflected in the same direction as our rudder input (i.e. toward the good engine), and stop the turning tendency -- which fundamentally is caused by the sideforce from the deflected rudder --  by banking slightly toward the good engine.

I slightly edit the question. An extreme case I'm interested in would be a constantly accelerating crosswind. The aircraft would be accelerating sideways relative to the earth (inertial frame of reference) but not relative to the airmass

I did not specified the pilot used the rudder. He uses any means to keep its flight coordinated. If there is no need to act on control surfaces (e.g. the airplane stay steady relative to the airmass such as a feather drifting into the wind), there is no need to act on control surfaces to keep flight corrdinated.

When you said "let's say the pilot is able to do it" I assumed you meant rudder.  You are right, you didn't say it.  When there is a CHANGE in wind then there IS a need to use some surface or other to keep the flight coordinated.  Anyway see first 4 paragraphs of my answer, are you now clear on how the yaw string and ball work in a STEADY wind?  Both stay centered with no need for special control inputs.

Yes for steady winds, but I should have insisted more since the beginning that I'm more interested in cases where wind is not steady and cannot be considered as an inertial frame of reference. (And yet, I'm learning quiet a lot about this indicator with your answer, that's really enjoyable)

@ManuH-- added fourth and eighth paragraph which you may find quite helpful.  Now that I better understand what you are asking I think the answer should probably have ended at the sentence reading "But if you feel ready for yet a few more twists and turns--"

"If the wind is initially zero, and starts blowing directly cross to the aircraft's original course, then as the wind gets stronger and stronger, before long the aircraft will be pointing 45 degrees off its original heading."  This is only true if the pilot makes continuous heading corrections to maintain a constant ground track.

@MichaelHall-- so you are saying an aircraft doesn't "feel" an increase in wind speed -- i.e. a gust-- in any way?  The vertical fin exerts zero yaw torque in such a case?

Sudden gusts are different.  See my other comments, the balance ball moves all over the place when it is gusty.  The question was about crosswind.  It is reasonable to presume steady state crosswind.  P.S.  I never ever thought I would see this much written about such a simple instrument...

@MichaelHall -- "It is reasonable to presume steady state crosswind"-- the sentence you quoted from my answer specifically said the wind was getting stronger and stronger.  Also even the original version of the question (and it is much changed now) referenced gusts.  Current version mentions the case of a wind getting stronger and stronger.  Even if the increase is very gradual, if it goes on long enough, the plane will get turned.

"I never ever thought I would see this much written about such a simple instrument."-- I never thought I'd see such horribly wrong diagrams in the study guides for FAA tests including FAA manuals about such a simple instrument.  Force vector diagrams that just don't add up, etc....Some of the test questions were probably messed up too, it's been a while since I've looked at them.

Well I applaud you for making an effort to clear things up for Manu, but to me this is equivalent to writing an essay explaining the physics on how using a carpenter's level to check the deck of a sail boat during high winds and rough seas could yield inaccurate results.  People either get it, or they don't...

"Write an essay" is one of the suggestion options for a stack exchange answer.  Actually I think his intuition that an accelerometer is referenced to a valid inertial reference frame such as (roughly speaking) the earth is worth noting.  So what happens when the wind field is changing?  There is more to the question that meets the eye.  "Beginners mind" may perceive something that advanced mind does not.

Yet there is such a thing as going overboard...  Personally I am a firm believer in simplifying explanations as much as possible.  The primary goal, after all, is understanding.  The scope and complexity of the answer should not exceed the complexity and sophistication of the question.  Rule number one in teaching is know your audience and teach to their level.  Ever hear the expression TLDR?

@MichaelHall -- the eighth paragraph is the real answer to the real core of his question.  I just didn't realize what he was really trying to ask, when I was writing all the other stuff.

I agree.  That paragraph could stand on its own, and as written is an excellent answer!

1. Answer to original OP
In a stable airmass, the amount of sideslip/skid indication is irrespective of the wind strength and orientation; it is a function of the relative velocity of the airplane against the airflow only (stable winds are referenced to ground). That is, with all engines operating and rudder centered (assuming no inherent asymmetry), then the sideslip indication will be zero, whether you have winds or not.
In gusts, the winds are transitory against both the ground and the airplane frame of reference, which will induce change in airspeed, AOA and sideslip. This will result in change in aero forces and moments, until the inherent stability of the airplane stabilizes the airplane or the gusts die down.
In a pure lateral gust, this translates to a sideslip (and an increase in airspeed) measurable on the yaw string, and generates a side force measurable on the skid indicator.  Both would concur in measurement (opposite in visual direction).
2. Addendum to address the modified OP

The air mass is no longer an inertial frame of reference.

No one ever uses airflow as a frame of reference unless you're doing fluid mechanics, which is not very relevant here. Airflow matters to airplane only because the latter responds to airflow changes (via aerodynamic forces).
The ball does not measure the crab angle; it's dumb and doesn't know what the airplane heading is or where it's going. The ball only measures the side force relative to the aircraft, much like a bubble level. With all engines operating (or no engine operating) and in flight, only aerodynamics can generate side force, most directly via sideslip (control surfaces can, too; but the magnitude pales in comparison).
In your scenario of a linearly increasing crosswind, sideslip increases with crosswind, measurable by the string. Increasing sideslip induces side force, measurable by the ball, whilst pushing the airplane off course, further increasing crab angle. At the same time, the airplane yaw stability responds by yawing the nose toward the crosswind. After some time, without pilot correction, the airplane has yawed sufficiently that the crosswind becomes a head wind.

What if during the gust the pilot react so quickly that there is no sideslip?  e.g. the aircraft is so light that it adapt itself to the airmass movement in no time like a feather (extreme case)?

@ManuH If you have a step increase in crosswind, by default yaw stability, the airplane will zero it out if rudder held constant (at which point there is neither sideslip nor side force). What you're describing is a yaw damper: it hastens the response and damps it (i.e. dutch-roll) out.

@ManuH The ball measures side force only (like a level). It doesn't have magic inside that knows where the aircraft is relative to the ground.

I thought it measures an acceleration (nothing relevant is pushing or pulling it). If so, the acceleration is relative to an inertial frame of reference (such as the ground), isn't it?

@ManuH Acceleration/force are interchangeable, so yes. But that still has nothing to do with crab angle.

acceleration are interchangeable only in intertial frame of reference. the airmass accelerating is not an inertial frame of reference whereas the ground is. acceleration must be defined in a frame of reference (speed is always relative and acceleration derived from speed). Thus there the ball is linked with any inertial frame of reference such as the ground, whatever the airmass movement (and thus the crab angle)

@ManuH Not sure what you're getting at :). Crab angle is the difference between heading and track. Think of a step increase in crosswind. At t=0+, neither hdng nor track has changed, but there's a step increase in sideslip. At t>0, this sideslip induces side force, which changes track (ball off-centre); due to yaw stability, heading changes as well to minimize sideslip. At t->infinity, sideslip->0, hdng and track settle to steady state. Now there's a new crab angle, but the ball would measure centered because there is no more side force present.

@quietflyer yes it kind of help, but there is still shadows. e.g. there is no movement of any instrument inside the aircraft relative to the aircraft (except if they are not fixed to the aircraft)

@Manu H, I am having a hard time understanding your confusion, and what you mean by "inertial reference". The balance ball is the simplest instrument in the airplane, and it doesn't know or care where the earth is. Nor does it react to wind. Have you seen the video of Bob Hoover pouring a glass of water resting on the glare shield of while he performs a barrel roll? (Google it if you haven't...) This is how the balance ball works; it moves due to centrifugal force. Period. It is unaffected by heading changes to keep a ground track due to wind, or anything else. It is completely dumb.

...it is hard because we don't know what your experience or base level of understanding is.  So just to be clear, the balance ball is quite literally a round ball in a curved glass tube, which is filled with liquid to dampen the movement.  Really that's it.  I don't mean to insult your intelligence in case you already knew that, but if you do understand how it works then I really don't know why you might think it is affected things like wind.  (other than the fact that it dances back and forth a lot in gusty conditions...)

... and if you ever get a chance to go flying sometime in a small plane ask the pilot to let you press the rudder pedals a few times and watch it move.  I think the light would come on pretty quickly when you see it in action and feel the corresponding forces on your body at the same time.


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What is the meaning of GPS IAP RNP that appears in Notes?
Referring to the RNAV (GPS) RWY 30 approach, what is the meaning of RNP APCH in the notes section? (Highlighted in YELLOW)
Also, what are the separate DA values in parenthesis? (Highlighted with a red oval)

The AC covering RNP APCH can be found here. Under RNP APCH Operations it lists:

6.3 RNP En Route, RNP Terminal, and RNP APCH Operations:

The pilot is not required to monitor ground-based NAVAIDs used in position updating unless specified by the Airplane Flight Manual
(AFM).

The pilot must comply with published and assigned altitudes, and airspeeds.

Note: Pilots operating aircraft with a barometric vertical navigation
(baro-VNAV) system must ensure compliance with all altitude
constraints as published in the procedure by reference to the
barometric altimeter.

The full qualifications can be found in APPENDIX A. QUALIFICATION CRITERIA FOR REQUIRED NAVIGATION
PERFORMANCE APPROACH (RNP APCH) OPERATIONS (not copied for brevity)
The landing minimums in the parentheses are non civilian minimums and generally for use by the military although the chart reference strictly says " All weather minimums in parentheses not applicable to Cilil Pilots. Military pilots refer to appropriate regulations" so there may be other qualifiers (Im not a military pilot and cant comment beyond this)

(source)

That AC was exactly what I was looking for: What is required of the pilot with RNP APCH in the notes. Thanks!!!

I also found the reference to the military minima in the AIM

source: Stack Exchange

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text:
What are the Radarless approach control services in the United States?
I was recently in Twin Falls, ID at KTWF. There is no radar for approach control services. All instrument approaches are full procedures and aircraft must report all requested fixes.
There is Salt Lake Center radar services at altitudes well above the usual approach and landing phase of flight for KTWF.
When I remarked to tower that is old school, they stated "last of two in the country."
What is the other?

"Radarless" = Procedural

Twin Falls, ID
Helena, MT

The Chart Supplement shows if a facility has radar or not. The entries for KTWF and KHLN show that there's no radar at the local TRACONs.

Did you go look through all of them until you found KHLN?

And since TRACON refers to Terminal RAdar approach CONtrol, is a procedural approach control still a TRACON?

@dawg No, it just happens that I saw a post on pilotsofamerica.com about this recently :-) I only checked to confirm that the Chart Supplement agrees. There are a few other Google hits too, although this seems to be very obscure.

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text:
Why don't airlines place some physical separator between passengers in the planes to prevent the spread of COVID-19?
Why don't airlines place some physical separator between passengers in the planes to prevent the spread of COVID-19?
A bit like the following, but adapted for cabins and viruses:

(image source)

Because a face mask is many, many times cheaper, much quicker to deploy, doesn't add any significant weight and doesn't restrict passenger movement (for example during an evacuation).

@expeditedescent thanks, I was thinking it could be in addition to masks, but downsides noted.

I'm sure I saw such a question few weeks ago but I can't remember if it was on travel.SE or on aviation.SE.

Also note how much "elbow room" there is in that image.  In my experience, at least, that's about 2 economy seats.

Good lord.  I wouldn't be caught dead in that thing.

Cost. Those would be expensive, and the business is bad already. Now, you might think that just any mesh, fabric or membrane would do, but but to be effective against the sperad of viruses, nope.
Discomfort. Imagine how it would feel to be bagged up like that for hours. Again, despite the membrane being a "breathing" sort, it would still get rather hot and huimid inside that contraption very quickly.
Restrictiveness. Imagine an evacuation situation with people in those. Very dangerous, not gonna work.

Masks are very effective, cheap, and people are quite willing to use them (excluding some mentally challenged cases). The ventilation in planes is also rather safe when considering the spreading of viruses (see this ASE Q&A), especially when combined with the use of masks. Planes usually have very good filtering of recycled air, and much of the air is not even recycled.

Time for evacuation would have to be retested

Need.  Planes get new air every 60-90 sec.


text:
Is Turn and slip indicator a good back-up for the attitude indicator?
I was reading my Aircraft general knowledge book, and noticed something peculiar that was written.
The book described that the turn and slip indicator works as a back-up in case of a malfunctioning artificial horizon / attitude indicator.
From my perspective this seems strange to me because of the following:

An attitude indicator shows pitch information.
A turn and slip indicator shows: "no pitch information".

Therefore the usage of a Turn and slip indicator as a back-up for the attitude indicator is not logical from my (probably narrow) point-of-view.
My Questions:
Is it true that a Turn and slip indicator is a valid back-up for the attitude indicator?
To what extent is a turn and slip indicator a back-up for the attitude indicator?

Is it true that a Turn and slip indicator is a valid back-up for the attitude indicator?

Partially, but it would need to be used in conjunction with one or more other instruments if the Artificial Horizon is inoperable.

Change in the DI and/or compass will indicate a turn.
Change in altitude on an altimeter will indicate a climb or descent. Usefulness of the VSI is limited in this sense.

To what extent is a turn and slip indicator a back-up for the attitude indicator?

With every one of the instruments on an aircraft there are backups. Sometimes this may be an entire second stack (eg Radio may have a complete second system, AoA sensors may be duplicated) other times you need to use a combination of more than one other instrument. In the case of a Turn/Slip indicator the latter is true. You  use a combination of DI, VSI, altimeter as described above to get a picture of your attitude when the Attitude Indicator itself is inoperable

It's called "partial panel" flying and if you ever advance to a commercial license, you will have to demonstrate proficiency at partial panel during training and on the commercial check ride.  On my commercial check flight in the late 70s I even had to demonstrate recovery from an unusual attitude (diving spiral) on partial panel under the hood because my examiner thought it would be fun.  Not sure they still make you do that these days.
Partial panel is maintaining controlled flight in IMC with only one gyro instrument, the turn and slip/turn coordinator, and pitot static instruments (you may be allowed to use the Directinal Gyro).  In a light aircraft the gyro horizon and Directional Gyro are normally vacuum operated and the turn and slip is electric.  So if the vacuum pump goes south, there you are with only turn and slip and pitot/static instruments (which is why the turn indicator is electric; to have an independent energy source).
In partial panel, the turn indicator is your primary roll source, airspeed + altimeter is your primary pitch source, and heading control is using compass and (assuming no Directional Gyro) stopwatch.
Because if its lagging indication, VSI is useless in partial panel work as a primary pitch source except as an indication of the longer term condition you are in, going level, up or down.
So, it's roll inputs to keep the turn indicator/coordinator centered, and pitch inputs to keep airspeed and altitude constant, compass to know your heading while in level flight, and stopwatch to change heading by timing the duration of the standard rate turn.  When you roll out of the timed turn, more or less in the ballpark on heading, you can then make very fine heading adjustments on the compass alone in little baby steps.
It takes finesse (make a small input and wait, make a small input and wait) and good skill at exploiting trim to fly well on partial panel, although once you get good at it, in a way it's easier because you have less instruments to scan.

The most common killer of inadequately trained pilots who fly into clouds is a spiral dive, caused by the aircraft entering a steep bank without the pilot realizing it or understanding the direction of turn.  Eventually the aircraft is destroyed due overspeed or excess G-load.
A magnetic compass is generally useless whenever an aircraft is banked and turning, so forget about trying to use that to keep the wings level, except in some very particular circumstances.  Hence the importance of a turn rate indicator or attitude indicator.
Many pilots have flown sailplanes in clouds with a turn rate indicator as the only gyroscopic instrument present.  Charles Lindbergh did a great deal of cloud-flying during his 1927 Atlantic crossing, and a turn rate indicator was his only gyroscopic instrument.  So the general answer to your question is "yes, a turn rate indicator is a valid backup for an attitude indicator".  But read on for more--
The turn rate indicator can tell you which direction you are banked, and can give you a clue as to how steeply you are banked.  However, with no attitude indicator, it can be challenging to manage the aircraft's pitch attitude.  Lots of practice in safe conditions is needed.  Flying with the turn rate indicator as the only available gyroscopic instrument is called "partial panel" flying, and pilots practice this in safe conditions outside of the clouds by covering up all the other gyroscopic instruments and donning a blind flying hood to block the view out the windows.
So how is pitch attitude controlled in "partial panel" flying?
If the airspeed is changing, that is a valuable cue to pitch attitude.  Inside a strong updraft or downdraft, you can't rely on the vertical speed indicator, or changes in the altimeter reading, to tell you the aircraft's pitch attitude.  Therefore you could argue that the airspeed indicator and the turn rate indicator are best pair of instruments to serve as a backup for attitude indicator, if you can only choose two and need to stay right-side-up in turbulence.  Of course the heading indicator also gives some clue of the turn rate and therefore the bank angle, and it also gives you heading, so that's a very nice instrument to have as well.  In some conditions, the heading indicator plus the airspeed indicator might be the best choice for a pair of instruments to stand in for the attitude indicator.
The relationship between airspeed, pitch attitude, and vertical speed will depend on aircraft performance as well as updrafts and downdrafts. A pilot flying a fast jet might have better results relying primarily on the altimeter for control of pitch attitude even in significant turbulence, while a pilot flying a light plane in the same conditions might have better results relying primarily on the airspeed indicator for control of pitch attitude.
Don't forget that the airspeed indicator can fail due to ice in the pitot tube, unless the pitot tube is heated.  That would be a bad thing to happen if your life depended on it.
If you are trying to use ONLY the turn rate indicator (or ONLY the heading indicator) to substitute for the attitude indicator-- i.e. if you are trying to fly using no pitch information of any kind, not even from the airspeed indicator or altimeter-- then you had better be flying a very pitch-stable, well-trimmed aircraft.  This has been done, but it has not always ended well.

Future edit: last paragraph could be modified to make it clear I am talking about flying in clouds, not VMC!

I openend a flight sim (X Plane 10) to check for similarities and differences between the turn and slip indicator and the attitude indicator.

If the airplane is horizontal then the airplane symbol on both instruments: turn and slip indicator and attitude indicator display similar data.

In a soft turn to the left, the airplane symbol on turn and slip indicator is tilted to the left. It almost reaches it's maximum. The airplane symbol on the attitude indicator tends to the "ground" on the left side.

In a sharp turn to the left, the airplane symbol on turn and slip indicator is also tilted to the left. But, it reaches it's maximum. The airplane symbol on the attitude indicator reaches the first white line on "ground" on the left side.

If the airplane is almost vertically, then the airplane symbol on turn and slip indicator is also tilted to the left. It doesn't deviate more then the maximum. The airplane symbol on the attitude indicator reaches the second white line on "ground" on the left side.
Conclusion: the turn and slip indicator and the attitude indicator show more or less the same angle on the roll axis. The differences are: turn and slip indicator is limited to a specific angle, the attitude indicator works beyond this angle.

I really don't see how this answers your own question at all. You've mentioned nothing at all about pitch, or how to use other instruments to back up a failed AH.

I guess I was asking the wrong question in the first place. I just wanted to see the difference and similarities between those two instruments... my bad. Anyway, your answer is the best answer to the question. I'll mark it as the best answer.


text:
Why Consumer Drones don't use multi-blade (more than 2) propellers?
Maybe unusual question but it's related to Aviation world, even if is more directly related to Drones one.
Why almost all consumer drones like DJI's and Yuneec's ones (but it's true also for other manufacturers) uses two blade propellers only?
I'd think that tri-blade or quad-blade could spin slower and so quiter.
Exist 3rd party multi-blade propellers compatible with commercial drones, so is possible to use them but what advantages/disadvantages they have? and again... why default ones are only two-blade?

I googled "3-blade propeller for drones" and got plenty of hits for sites selling and folks using them and discussing using them, so I'd say your premise is incorrect.

The more blades you add, the less efficient you get. More blades is helpful if you have the power to drive them (or need to spread the power out), but ideally you'd want as few blades as possible for the best efficiency.

In the world of full size planes, two blades will deliver a faster cruise speed but three blades will deliver better acceleration and climb performance. The assumption here is that an engine is not so powerful as to REQUIRE three blades...only that a two or three blade option exists (think IO-520). The performance gains in either direction are so minor that most pilots make a prop decision based on how it looks or how the extra weight changes the W&B.

There are a number of related questions on the site for more information about this. I'll link them below.
The short answer is that two blades will be the most efficient use of energy. If the drone needs a better climb rate then adding extra blades is an option, since you can't make them any longer. The downside is that it will reduce efficiency and thus your battery time.
I can't find the question at the moment, but Peter Kämpf mentioned in an answer that theoretically one blade would actually be most efficient except that you would have to provide counterweight to balance it. The counterweight with no excess thrust would negate the advantage, but that's just an illustration of how the fewer blades the more efficiency.
Here are the links for more reading:
What's the difference between a two-blade prop and three-blade?
Is a propeller with two long blades more efficient than one with three shorter blades?
How does blade solidity ratio relate to thrust/power/torque of a propeller?
Why are three-bladed helicopters relatively rare?

Here's a quadcopter with single-blade props: https://www.youtube.com/watch?v=ab8Dff8X5j0

@Steve They managed to make it fly. They didn't mention if it was truly more efficient or not. With a duct taped prop, probably not though.

It's the only example I know of :)

I think the answer you were looking for is this one: https://aviation.stackexchange.com/a/23015/3394

If the weight of a 3 blade prop is equal to or less than the weight of a two blade prop you're replacing and the pitch is equal to or greater then the 2 blade prop to increase thrust proportionally then theoridically it will give better overall performance without sacrificing battery life. With the 3 blade prop made of a strong super lightweight material  in theory it is possible.

Do you have something you can cite for this? The most efficient propeller is a single blade. More blades generally reduce efficiency.

I can say for a fact that 3 blades are slower in cruise than 2 blades, despite a specific example of a composite 3-blade prop being 11 pounds lighter than an aluminum 2 blade prop.  There is a Cessna Cardinal forum where this has been demonstrated on several planes with 180 and 200 HP engines. The 3-blade does accelerate faster on takeoff and improves climb rate some (perhaps 100 ft/min better), but once at cruise is slower. Also increases drag  lot when in the pattern and slowing down for a landing.  I can't say how much this carries over into RC craft.


text:
What pitch attitudes are common during an emergency descent for an airliner?
What pitch attitudes are common during an emergency descent? I assume it would be a fairly nose-down attitude, but I can't imagine how steep would feel to the passengers.
Suppose a gradual decompression takes place and no structural damage is present nor suspected. What would be the pitch attitude for common airliners like the Airbus A320 or the Boeing 737?

fun video on this https://www.youtube.com/watch?v=yHawjB2PzK0

@Cloud Nice video! If you look carefully at around 2:40 they start descending. You can just about see the vertical speed on the PFD going down. Note how smooth it is :)

It is not as nose down as you may think. First, let us have a look at normal pitch attitudes during a normal descent (without speedbrake). These can be found in the QRH Performance Inflight Tables:

(Boeing 737 NG QRH - 30.1 Performance Inflight - Flight With Unreliable Airspeed)
As you can see, normal descent attitudes are varying between -2.5° and 2.0° depending on aircraft weight and altitude.
For an emergency descent you would extend the speedbrakes which allows a steeper descent. The FCTM has some details on the procedure (see also this answer for more details):

To manually fly the maneuver, disconnect the autothrottles and retard thrust levers
to idle. Smoothly extend the speedbrakes, disengage the autopilot and smoothly
lower the nose to initial descent attitude (approximately 10° nose down).
About 10 knots before reaching target speed, slowly raise the pitch attitude to
maintain target speed. Keep the airplane in trim at all times. If MMO/VMO is
inadvertently exceeded, change pitch smoothly to decrease speed.

(Boeing 737 NG FCTM - 7.7 Maneuvers - Rapid Descent, emphasis mine)
Note that the 10° nose down attitude is only the initial pitch to start the descent and increase airspeed to MMO/VMO. Afterwards, the nose is raised to maintain the speed limit. This means the pitch will be between -10° and -2.5° during most of the emergency descent. Enough to notice it, but hardly the nose dive some Hollywood movies show.

I disagree. I think you'd experience zero G in an emergency descent

@Cloud For sure not. Note words like "smoothly lower the nose" are chosen in the Boeing documents. Basically, you should not rapidly start the descent. The Zero G flights (the intentional ones) get zero G by flying on a parabola to achieve this (meaning you get zero G long before they actually descend).

By all means, @Cloud, please write up an answer of your own referencing documentation of a procedure for a major commercial airliner that would produce a zero-G emergency descent.

@Cloud you may experience a low load factor when putting the aircraft in a dive but then you rapidly reach max speed and continue on a straight path at constant speed (no acceleration and thus you experience 1G), but zero-G seems to be an extremely low load factor.

If I read this table correctly, an emergency descent from 40,000 feet at a weight of 80,000Kg requires a pitch attitude of 0.0° and generates a -3400 ft/min vertical speed? What the heck pitch attitude does one fly at at that altitude and weight to maintain a constant altitude??

@FreeMan Careful, the table is for normal descent (no speedbrake). To answer your question: maintaining 40,000ft at that weight is not even possible. The cruise table in the QRH lists a pitch of 3.5° for 60t weight for 40,000ft and also a pitch of 3.5° for 80t weight at 35,000ft (which is about the max. altitude for this weight).

Thanks for the clarification. I presumed, since you presented it in reference to a question on emergency descent and the graph labeling says "Flight With Unreliable Airspeed", that this was for emergency descent. Still though, 3400ft/min descent rate is pretty decent...

@FreeMan: it's not as much about pitch as it is about having engines. If you are doing en emergency descent, first (or second) thing you would do is retard the engines. Also, at 40000ft, with 180kt IAS (so about 360TAS, say), that rate of descent corresponds to an L/D of about 11:1, which sounds about right for a jetliner at idle.

@Cloud Remember that what you can feel is acceleration, not vertical speed itself. Even if you're descending at 10,000 feet per minute, if it's a constant 10,000 fpm, you will feel 1 G just like you would in level flight. There's absolutely no need to accelerate downward at 1 G (what is required to feel 0 G) to enter an emergency descent. That would be an acceleration of 1,920 feet per minute per second. So, 5 seconds of that and you'd have a vertical speed of nearly -10,000 fpm. Extreme maneuvers to shave a few seconds off of a minutes-long descent aren't necessary.

"the nose dive some Hollywood movies show" is often made even worse by many movies adding the Stuka siren sound effects (which in real life was a device deliberately added to make that sound, in order to frighten the enemy), so many people associate any aircraft which is rapidly descending with that loud wailing.

My experience was on the Regional Jets but from sim training I recall that the pitch down once at Vmo with flight spoilers extended was somewhere around 10-15 deg.  The standard procedure for the CRJ was to declare "emergency descent", slam the thrust levers to idle, flight spoilers fully out, dial the Autopilot altitude preselect to 10000 ft, select the Autopilot to Speed Mode, and dial the speed select to Vmo, and just let the airplane do its thing while the two of you went through the QRH.
You will get a small G reduction during the pushover, but the AP would pushover gently enough to avoid making people in the back float around.  If you push over manually, there is a risk of pushing too hard in the stress of the situation and banging heads against the ceiling in the back (it doesn't take much when you're going fast like that).
Any G reduction that occurs is only momentarily during the pushover.  Once up to speed you are back to 1 G more or less. The pitch attitude in the descent would be noticeable to a passenger looking out the window from the slanting horizon, but I don't think it would look like they were diving straight at the ground.  The scarier part would be the rumble from the spoilers being out, and the roar in the cabin from flying at Vmo.


text:
Do airliner cabin filters trap viruses or other pathogens?
I understand that half the air in an airliner is re-circulated (per How much air is recirculated (vs. bleed air injected) in modern airliners cabin?). Of this 50% re-circulated air, what it's the chance that viruses pass through its filters?
Disregarding possible recent developments due to the COVID-19 outbreak, were filters normally designed to remove viruses or sterilize the air somehow?

General coverage by Juan Browne AKA blancolirio.

Modern airliners use HEPA filters to filter the air before re-circulation. This will remove almost all airborne pathogens like viruses:

The overall risk of contracting a disease from an ill person onboard an airplane is similar to that in other confined
  areas with high occupant density, such as a bus, a subway, or movie theatre for a similar time of exposure.
  anywhere where a person is in close contact with others.
That said, the risk on airplanes is probably lower than in many confined spaces because modern airplanes have
  cabin air filtration systems equipped with HEPA filters.
HEPA or high efficiency particulate air filters have similar performance to those used to keep the air clean in
  hospital operating rooms and industrial clean rooms. These filters are very effective at trapping microscopic
  particles as small as bacteria and viruses.
[...]
Air filters can remove very small particles such as bacteria and viruses. Virtually all viruses and bacteria are
  removed; even the most difficult particles in the range of 0.1 to 0.3 micron are filtered out with an efficiency level
  of of 99.995%. Contrary to popular belief, very small particles below 0.1 micron are easily filtered out by the
  mechanism of diffusional interception.

(IATA Briefing Paper - Cabin air quality - Risk of
communicable diseases transmission, emphasis mine)
The following image shows the location of this filter in front of the re-circulation fan for a Boeing 737:

(Boeing 737 NG FCOMv2 2.30.5 Air Systems - Air Conditioning System Description)

IF the airliner has HEPA filtration yes, so pathogens floating in the air that happen to get drawn into the recric system would be trapped and not recirculated, so some of the total pathogen load in the cabin, but not all, would be removed.  In theory, if you just added a fixed amount of pathogens at the start and just let it run, the concentration would slowly decrease.  Problem is, if there are sick pax on board they are continuously issuing them, so the filtration is only a mitigation.
If an airplane doesn't have HEPA filtration, effectively, forget it.  There are HEPA elements made for older airliners with existing recirc filters under PMA that are a direct replacement, but you won't know which airplane you're on has them.  Maybe you can ask when a book a ticket.
In any case, I said "only a mitigation" because the pathogens you are going to be exposed to on the flight are all around you in the cabin and what may be removed by the recirc system really isn't going to help you all that much.  If you catch something it's usually because you touched a contaminated surface and then touched your face at an entry point, like the eye, after.
This is why wearing face masks can help in preventing breathing in a pathogen directly, but you usually don't acquire them that way anyway.  When you catch a cold from someone in the office, it's rarely because they coughed in your face; it's because you touched something they touched, or coughed on, earlier and then rubbed your eyes or scratched your nose.
People who are very disciplined about never touching anywhere near their faces with their hands probably have the best protection from catching these kinds of things.

Well done for staying with the real question.

Most face masks people are wearing do not protect from anything breathed in.  They do help a lot in protecting others from what you breathe/cough/sneeze out.

Yeah pretty much.

@Harper-ReinstateMonica the "real" question was about airliner filter systems. This looks like general health advice and (albeit undoubtably correct health advice) doesn't answer the question at all.

I would reword that to avoid starting the second paragraph with "This is why wearing face masks can help", as a lot of people will just stop reading there.

Now that information has been added directly answering the question, the last two paragraph seem pretty tangential and distracting. Even though they made up most of the content of the original answer, I think it would improve the answer's quality to remove them.

@Will I’ve submitted an edit. Aside from not directly answering the question, it’s also unsourced (and slightly misleading) health advice.

@Tim if it makes you happy I've added a link to source the blindingly obvious "health advice" (not touching your face - what my mother told me 55 years ago) and HEPA technology.  Also I was a ECS specialist for many years in the regional airline business.

@JohnK I’m more concerned about the comment around masks - they really don’t work. Either way, its OT and not appropriate on this site.

It's worse to let people think that masks protect them from catching things.  As I said, they will protect from direct inhalation, but that's only a part of the risk of catching stuff.  Masks are mostly to protect everybody else from the infected person, when the infected person wears it.  It's on topic because it relates to the underlying premise of the question, which is concern about catching something on an airliner.


text:
Is it allowed to start engines during push-back?
Is it allowed, considered safe, and done, to start the engines while the aircraft is moving under the push-back power? On one hand it seems like a big time saving, on the other hand the running engines are rather dangerous and there is still a wing-walker often present during the push back.

I don't know about these days, but back in the 1990s, we often did it. But then we had 3-man cockpits.

It is common. In fact, for aircraft parked at a gate (i.e. not a hardstand), it is quite uncommon NOT to start the first engine during pushback. The wing-walkers stay well away from the intakes, and with the engines at idle power, the danger zone isn't all that large - not nearly as large as if the motor were running at a high power setting.


text:
Is there any aircraft that cruises in the transonic regime?
Has there been any airplane that has been able to cruise in both the subsonic and transonic regime where the drag is high because the critical Mach number has been reached or surpassed?

If we define the critical Mach number as "the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it," then yes, many aircraft cruise above this speed. Any modern jet airliner has some supersonic flow over parts of the wing and fuselage at cruise, placing it squarely in the transonic range. This video and its description provide a great visualization and explanation of the associated phenomena.
It should be noted, however, that there is not necessarily a large increase in drag upon reaching the critical Mach number. More useful in the transonic range is the drag-divergence Mach number, which is always greater than the critical Mach number. Supercritical airfoils are designed specifically to increase the drag-divergence Mach number, and are fitted to all modern jetliners.

@Peter gives a quite clear explanation. I would like to add that the speed range of 'subsonic' and 'transonic' actually overlaps with each other, for subsonic flight, the speed is less than Mach 1, whereas for transonic flight, the speed the between Mach 0.8 and Mach 1. Nowadays, most commercial aircraft fly at a speed between Mach 0.8 and Mach 0.88, well with the transonic range.

It's true enough that the literal definition of "subsonic" does mean "less than the speed of sound," but that's not the same as "subsonic regime," which ends at the beginning of the transonic regime. See Wiki: Mach regimes.

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text:
Why are the largest civilian aircraft not adapted for military use?
Frequently, civilian aircraft models are militarized, e.g. into the role of tankers, AWACS and others.
In this militarization, we see a lot of "medium-large" aircraft, such as B707, A330.
The super-large models, B747 and A380, however, do not see militarization. The current 747-based Air Force One, comes to mind, but that's more or less a one-off.
What are the reasons for not seeing more B747s and A380s being put into a military role?
A huge B747 or A380 would make a quite extensive flying gas-station.

Consider that civilian aircraft get to land on long, smooth runways.  Military transports often don't have that luxury.

Even with an A380, you can still only fuel up two aircraft at the same time. Better to buy several (still comparatively huge) A330s and fuel four at the same time.

I explicitely did not list transport as examples, as I am well aware of the militaries prefering rugged aircraft for forward operations. AWACS and tankers, however, mostly operate from well prepared fields and are usually well protected.

Indeed takers are gas stations, so we need a high pump/money ratio, or refueling rate / money ratio. Practically speaking there's no way you can fit 4 hoses on a A380, so multiple smaller tankers with 2 hoses each is more cost effective.

Boeing 747s do see extensive use as transports in military operations. It's just that plenty of capacity is available on the commercial market, so there is no need for militaries to maintain their own fleet. (Recall the National Airlines Flight 102 crash) Same thing with large passenger-only transports.

The 747 is used as a tanker by the Iranian air force.

747 was considered for use as a heavy duty military transport but lost the competition to the C-5 Galaxy (which itself lost its bid as a civilian airliner to the 747).

There are several angles to this question, so I'll try to address what I think is important. My so-far short career has so far involved working almost exclusively with 747 derivatives, so I don't have as much to say about the A380, but I'll touch on it as I can.
In summary, the decision to modify a commercial aircraft for military use comes down to cost, schedule, performance, and politics. The platform needs to be the right one for the mission, be available in time to execute the mission, be worth the money for its capabilities, and be politically appealing. These four factors are best addressed on a case-by-case basis. The 747 has a much longer and more storied history than the A380, and I am not aware of any thoughts to militarize the latter; however, the 747 has been adapted for military use in several circumstances and there have been even more preliminary studies about doing so that never made it to production. So here is a brief summary of those programs. Hopefully looking at each will satisfy your curiosity and give a picture that answers the question.
Civil Reserve Air Fleet
Adapting an aircraft for military use does not necessarily involve modifying it in any way. The United States has the Civil Reserve Air Fleet (CRAF), a program through which domestic airlines pledge the availability of some of their aircraft to the Department of Defense "to augment the Air Mobility Command's long-range intertheater C-5s and C-17s during periods of increased airlift needs." Currently, none of these aircraft is permanently modified. Perhaps the interior of a passenger model gets reconfigured for a CRAF flight, but they are the same airplanes that members of the public fly on regularly. Back when Delta and United flew 747s, nearly all of them were available to the CRAF; now it is only 747 freighters that are available. The cost-schedule-performance analysis of this setup is favorable. The DOD only pays for the airplanes when it needs them, and is legally allowed to use them any time it needs them. Airlines get compensated at above-market rates when their aircraft are used. And unless you need to land on an austere runway, fly through contested airspace, perform an airdrop, or load/unload extremely quickly on the ground, a 747 can do almost anything that a C-5 or C-17 can, and usually more efficiently.
However, in the late 1980s and early 1990s, about two dozen early-model 747s were structurally modified specifically to support the CRAF, including N739PA:

The fuselage structure of N739PA differed from the majority of Boeing 747 aircraft in that it had been modified to carry special purpose freight containers on the main deck, in place of seats...The effect of this modification on the structure of the fuselage was mainly to replace the existing main deck floor beams with beams of more substantial cross-section than those generally found in passenger carrying Boeing 747 aircraft. A large side loading door, generally known as the CRAF door, was also incorporated on the left side of the main deck aft of the wing.

The CRAF door can be clearly seen in this photo:

Again, the analysis is favorable. The modifications were relatively simple structural ones and the DOD paid an ongoing subsidy to operators of these aircraft to offset the cost of the extra weight, which was much less expensive than maintaining a fleet of military-only aircraft of comparable capability.
I do not know what capabilities there are for NATO or other European countries to press civilian aircraft into service, but they do not seem extensive. This is where I think you'd most likely see an A380 used in a defense role.
Airborne Aircraft Carrier
Here's a fun one that didn't make it very far. The idea was to use a 747 as a flying platform for the launch, recovery, and refueling of small fighter aircraft:

Boeing's report sung the praises of this idea, of course, but my feeling is that it was something of a solution in need of a problem. The USAF had experimented with the idea before and found it to be technically feasible but operationally difficult, which was likely again the case here. Although the proposed schedule was favorable, the performance would not have matched a mission, and the cost of putting the system together and deploying it would likely have been huge.
Advanced Tanker Cargo Aircraft
The 747 was indeed converted for use as a refueling tanker, albeit in small numbers. The KC-10 won over a 747 variant mainly due to lower costs and better overall performance. The KC-10 could operate out of smaller airfields and had an all-new boom, as opposed to the 747 contender's barely-modified KC-135 boom with poorer handling qualities and less clearance away from the aircraft. There were a few other technical aspects favorable to the KC-10 as well, not to mention the inherent ongoing cost savings of flying a trijet over a quad.  Additionally, McDonnell Douglas at the time was in need of a lifeline for the DC-10 program and thus applied more political pressure. It should also be noted that the 747-based tanker design was better suited operationally for refueling long-range bombers and transports than fighters. As the other answers and some comments point out, an enormous tanker has some inherent deficiencies when directly supporting combat operations. My opinion is that Boeing focused too much on the long-range mission when trying to sell their solution, but this might be the best example of the 747's size as a direct and insurmountable detriment, whereas in most of its other military roles its size is its greatest benefit.
Despite the US not adopting the 747 solution, Iran purchased a couple of them, and at least  one still flies:

For Iran, the cost-schedule-performance triangle was beneficial. My guess is that Boeing offered a discount to Iran to get some tankers operationally deployed to help boost their case with the USAF, they were available almost immediately, and Iran's military operations were more concentrated and less reliant on situations that would put such enormous links in peril.
And just because it's interesting, it was the very first 747, RA001, that was modified to test this refueling concept, as seen here:

RA001 still boasts most of that equipment (the boom is long gone, though), but it should be noted that there were no modifications to the fuel system to allow it to pass fuel—the hardware was for testing tanker-receiver contacts and boom dynamics only. You can see RA001 up close for yourself at The Museum of Flight. And if you do stop by, say hi—I'm one of the volunteers on the restoration crew for this airplane.
Cruise Missile Carrier Aircraft
Another interesting one that never made it past the drawing board, although it got farther along than it usually gets credit for. The idea was patented by Boeing during the B-1 program as a way to supplement that aircraft and ease the B-52 into retirement.

It was eventually canceled in favor of the B-2 and upgrades to the B-52. History has questioned that decision, but at the time costs were deemed too high given commitments to the aforementioned platforms, and the considerable performance was still not found to fit well into the contemporary operational concept.
E-4
The E-4 was the first 747 derivative to enter military service. There is not much to say about it technically that Wikipedia doesn't, but it is worthwhile to note that, in the view of the Air Force, having four engines is a must on this type of platform. In the early 1970s, there wasn't much by way of twins, so that was taken care of at the time of aquisition. Redundancy and survivability are primary design drivers, so although its systems could probably have been transferred to a smaller, cheaper airframe, there is no way to match the mission assurance of four engines. Along with that, it would be tough to find a way to generate enough electricity on the airplane to power all its systems without four engines. Additionally, the first two airframes were offered to the government at a discount due to their being built for a canceled order.
Non-Developmental Airlift Aircraft
More a case study than anything, this might actually be the best example for how cost, schedule, and performance are balanced. I highly encourage you to read this retrospective:

...[the NDAA] was originally developed in the early nineties due to poor cost and schedule performance during phase two...of the C-17 acquisition. Forty C-17s were being built for phase two; and due to problems, alternatives were sought with respect to the remaining 80 aircraft. Attempting to leverage commercial industry, the NDAA promised a low-cost alternative to the DOD acquisition process. However, since this was an alternative for the C-17, DOD stipulated that the NDAA would have to be a jumbo cargo aircraft capable of carrying outsize cargo. The NDAA study focused on a minimally modified Boeing 747 cargo aircraft. These “minimal” modifications included hardened decks and a flipup nose and ramp system for ease of straight-in loading versus the sidemounted-cargo-door style loading of the commercial industry. Consequently, the price for the NDAA alternative increased from under \$150 to about \$200 million per aircraft. Nevertheless, after examining several options, the most cost-effective solution was an 86/30 mix of C-17 and NDAA aircraft. This mix, however, did not allow for a full strategic brigade airdrop nor was it optimized for tactical airlift requirements and lesser regional contingencies in support of peace enforcement scenarios. These requirements were better met by the military-specific C-17. Ultimately, the Defense Acquisition Board decided to procure the fleet of 120 C-17s and no NDAA.

Airborne Laser
The YAL-1 ultimately suffered from cost, schedule, and performance problems. The idea was to fly around a huge laser that could heat up and destroy ballistic missiles from afar. The 747 platform was chosen because it was the only airframe large enough to contain the huge tanks of chemicals needed to generate a laser strong enough to have any effect. But after almost two decades and more than five billion dollars, it still needed a laser many times more powerful to be operationally viable, so it was canceled. One prototype flew for over 10 years, but was unfortunately scrapped.

VC-25A
The VC-25A is generally what people think of as Air Force One, although technically that designation applies to any USAF aircraft with the president onboard. Again, the number of engines was a major design driver, and in this case the size and iconography of the 747 helped it tremendously. The DC-10 was competed as an alternative, although it never was a serious contender and was reportedly only included to get Boeing to lower its price. As with the E-4, survivability, redundancy, and power requirements support a four-engine airplane; the floor space of a 747 also supports a lot of functions that couldn't otherwise coexist. Even by the time the VC-25A was introduced, the 747 had an enduring and recognizable preeminence in the history of aviation and Unites States prestige that the government found attractive to its branding as the president travels the world. Commonality with the E-4 airframe was also a consideration. In this case, politics and performance played an outsized role in selection of the aiframe, with cost and schedule secondary. Obviously there was much willingness to spend the big money and wait some extra time to fulfill all the requirements.
VC-25B
Once again, we have power and redundancy requirements driving the selection of the airframe for the next presidential aircraft. There was some early talk about perhaps pursuing an A380 option, but politics quickly put that idea to bed in favor of the 747-8. The VC-25B will have modifications similar to those of the VC-25A, and will probably look similar:

The program has been much in the news due to cost, schedule, performance, and politics, but is overall proceeding with similar priorities as the VC-25A program did.
Conclusions
I count about 30 747 airframes that have been modified and flown for military purposes. Obviously that's not a huge number, but it is actually fairly respectable for an aircraft of its era, although 707 variants are clearly more ubiquitous (it's important to note, however, that the C-135 family is not a derivative of the 707). Nowadays, with the KC-46 and P-8 leading the way, commercial derivatives are becoming more attractive to the armed forces. But with production of the 747 winding down, we are unlikely to see any more variants beyond the VC-25B: parts will become more difficult to obtain and the labor force skilled with the airframe will dry up. The 747's size and capabilities have both helped and hindered it along the way, and it warrants a case-by-case look at each program to see how cost, schedule, performance, and politics have played a role. The A380 is a somewhat different case, being more recent and less revolutionary in its era than the 747. It has not found a military mission that cannot be accomplished with smaller, cheaper airplanes, and it has no history of modification to help it win over the decision-makers. I am doubtful that we'll ever see it take on a military role.  A lot more could be said about any of my examples and the overall picture, but I hope I've dispelled the idea that the 747 is not used militarily, and I hope I've provided some insight as to why it is not used more.

Thanks for contributing to the MoF in Seattle. I interned at Boeing 2 years ago, and I visited the MoF thrice and Randy's diner twice that summer. Aviation heaven!

The CRAF door became a commercial feature as later versions were sold as the 747-200C/M Convertible/Combi and 747-400M Combi. KLM is the major carrier that still flies them.

Chances are that NATO wouldn't have any agreements with civilian companies, but rather the member nations. I know that at least Norway had laws regulating the military use of civilian assets during the cold war, and it seems they are still valid (in Norwegian). I'd expect that most other European countries have similar agreements.

Regarding A380, it is already going out of production and a freighter variant was never built. Apparently the problem is that it has a lot of internal volume, but not the maximum take-off weight to match, so one can't actually utilize the volume on long routes and therefore the plane is not very efficient for cargo.

Ad the RA001—the SR-71 used different fuel than any other aircraft, so it couldn't be feed off the main tanks. Did they add a separate tank for the tests?

@JanHudec The tests with RA001 did not involve any fuel transfer to any aircraft. Just "dry" contacts to explore the tanker-receiver-boom dynamics.

Fuel mileage matters for flying gas stations
One of the big knocks on quad-jet widebodies (and why they're falling out of favor these days) is that the extra engines lower the fuel economy of the aircraft compared to a twin-jet.  While I don't have precise fuel consumption figures to back this up, this problem is especially acute for a tanker, as not only does having poor gas mileage limit your on-station time, it limits your ability to offload fuel to other planes, as tankers generally are providing fuel from their own fuel supply.
Big plane, big wake, big problem for little fighter jet
The other reason you don't see tanker conversions of such large aircraft (especially the A380) is because their wake turbulence is excessive compared to a smaller widebody, placing unreasonable demands on fighter pilots trying to fly formation behind the tanker.  Having the roll rate of a F-16 does you not a whit of good when your entire wingspan is engulfed in a wake vortex!

In the case of tankers, you have to keep in mind their purpose: to refuel combat aircraft engaged in combat.
It is quite possible that a sophisticated opponent will go after the tankers, as an easy way to disable a lot of aircraft without actually having to shoot them down. Doesn't really matter if they go down from a missile hit or lack of fuel. The math on that one looks very good - shoot down a tanker, and you have probably shot down four or five smaller, agile, hard to hit strike aircraft.
Put simply, one large tanker means that if it is hit, a lot of planes will be out of gas. Better to have two or three midsize ones, so that if one gets hit, you haven't lost all of your refueling capability and your strike aircraft aren't left completely dry. To be fair, tankers usually have a pretty good CAP guarding them, but still... one lucky hit one one big tanker could be a real problem.
Even in the absence of a direct threat, large tankers aren't as suitable for combat refueling as smaller ones, for the same reason that very large airliners aren't as cost effective as smaller ones today - greater flexibility. Most aerial conflicts today tend to be smaller regional affairs. Several midsize tankers can cover several regional conflicts, whereas large tankers can service only one area.

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How does the thickness of a wing affect its aspect ratio?
As far as I understand, the aspect ratio equals the wingspan divided by the chord or the square of the wingspan divided by the wing area. Given this formula factoring in only the wingspan, the chord, and the wing area, how does the thickness of a wing increase or decrease a wing's aspect ratio? If possible, please explain in plain English without using too complex mathematical formulas.

Indeed, the aspect ratio is defined as you describe. It is a way to define how slender the wing is for a given wing area, which is the main parameter in determining how to generate enough lift to keep the aircraft aloft. An area is two-dimensional, and a broadly rectangular shape like a wing area is defined by the length and the breadth of it: span and chord.
Aspect ratio describes the proportions of span and chord: a short stubby wing, or a long and slender one? This is one of the main parameters in how much drag the wing creates: a long slender wing minimises one particular form of aerodynamic drag, called induced drag.
 Image source
But a wing is a cantilever beam, subjected to aerodynamic lift. The lift forces bend the wing upwards, and the wing structure must produce internal forces that cancel out the aerodynamic lift. In a hollow cantilever beam like a wing, the skin mainly does that: the upper skin resists compression, and the lower skin resists tension. The further away the upper and lower skin are, the lower the stresses. A structural engineer wants to give as much distance between the plates as possible: he wants the wing to be very thick, in order for it to be light in weight.
But again aerodynamics puts a limit to this: a very thick wing would create too much form drag, so the structures people are given the maximum distance that the aerodynamics can support. Wing thickness affects aspect ratio in defining the maximum AR that can be achieved.
The wing thickness is not part of the induced drag, and does not appear in the definition of the aspect ratio. It is an important defining parameter in wing weight and other forms of drag.

Thank you for your reply, but I'm still confused. So according to your explanation, the wing thickness doesn't affect the induced drag, and therefore it doesn't appear in the definition of the aspect ratio. Then, was I wrong to believe the wing thickness affects the aspect ratio in the first place? Is the wing thickness nothing to do with the aspect ratio per se?

@lemonincider Yes indeed, the wing thickness does not appear in the definition of aspect ratio. It only imposes a limit on the maximum aspect ratio that can be achieved, at a given engineering standard.

That clears my confusion. Thank you very much!

Your mathematical understanding is correct: aspect ratio is theoretically independent of thickness. However, a case could be made that in some circumstances material and manufacturing restrictions play a role that affects both thickness and aspect ratio. For example, a very long wing with a short chord length may need more thickness to support its length than a wing of the same length but a longer chord.


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What is significance of N1 and N2 in Jet engine?
Why N1 Low pressure compressor RPM and N2 High Pressure compressor RPM are monitored in jet engines ?
Why specifically compressor RPM rather than turbine RPM ?
What do N1 and N2 signify in terms of relation with any engine performance parameters?

Try this search: is:q n1 n2 to find questions relating to N1 and N2.

Are you talking about a pure jet or turbofan?

I am referring to Turbojet engine

Compressor and turbine are connected by the same shaft. You have LP compressor+turbine and HP compressor+turbine. For Rolls Royce engines you have N3 too because they use three rotors instead of two.
N1 or N2 show the percentage of rpm for the respective shaft. Each engine has a particular number of rpm corresponding to 100% N1 or N2 in order to make numbers easier to understand for pilots between different engines.


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What type of aircraft do Tintin and Captain Haddock use on their way to Peru in "The Seven Crystal Balls"?
At the end of Hergé's The Seven Crystal Balls (first published 1943-1944), Tintin and Captain Haddock fly to Peru in a relatively big seaplane (or flying boat?).
Is it a real aircraft? If so, what type is it?

EDIT: After finding the answer, I am adding here the picture from mins' comment, because it deserves being more clearly visible:

The small portholes make it look much bigger

@Klaws Oh, you are right! I perceived the portholes as "standard" passenger aircraft windows.

It bears a distinct resemblance to a Short Sunderland, except that it lacks the gun turrets. The shape of the hull, the fin, the engine configuration, the cockpit, the forward observation window (a gun turret on the original), the wing shape, even the general pattern of the windows match pretty closely given that it's a drawing not a photo. The tailplane in the drawing is more swept than the original.
Sunderlands  converted for non-combat use were called Hythes.
This astonishingly complete list of vehicles in Herge works confirms it. (Thanks mins)

This of course assumes the illustrator had a particular airplane in mind, rather than just a general seaplane.

@EugeneStyer Hergé was very realistic, I am not sure if he ever "invented" any vehicle or aircraft.

@EugeneStyer , This of course assumes the illustrator had a particular airplane in mind, rather than just a general seaplane Of course, in some cases cartoonists draw an abstract general "car", say.  (A "cartoon car".)  But in other cases, they very specifically draw a very specific car! In this case it is (look at the drawing!!!!) INCREDIBLY rigorous and precise!! BTW a great example is Ghibli's Wind also rises which features BOTH insanely perfect, staggeringly perfect drawings of certain aircraft, and also, totally bizarre nonsensical "cartoon aircraft" (in the dream sequences).

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If a tail dragger is converted to a tricycle gear airplane, what will be the effect on weight distribution?
If a tail dragger airplane is converted to one with conventional tricycle landing gears, how will this affect the balance of the airplane? It is possible to offset the effect by shifting weights around the airframe?

Is this a homework or exam question?

Generally, we don't answer homework questions without more information from you. You need to show what work you have done, and explain what it is you don't understand. Personally I don't understand the question so unless you can edit your question and explain more I can't help. Sorry!

Taking an existing taildragger and redesigning it for tricycle gear use would be a significant engineering job, it would need more than just weight and balance checks.  (For example, changing the position of the main wheels, stengthening those new positions, checking for propeller strikes or tail strikes etc.)  That may be a good start.  (Note some small aircraft are available in multiple versions so redesign is possible.)

So what will happen to the datum and moment? Sir andy

An aircraft's reference datum is an arbitrary fixed plane, so nothing will happen to it in a design modification unless you remove some part by which the reference datum is defined (e.g.: you move the main landing gear struts and the reference datum is defined in reference to their former position.)

This question should be narrowed to specific aircraft such that the Me 262 whose prototype did not feature a tricycle landing gear.

There are some planes out there that exist in both configurations. One of the more common and first that comes to mind is the popular Cessna 150, the Cessna 172 also has an available conversion.
As a taildragger

(source)
And with tricycle gear

(source)
Since the majority of the mass remains unmoved (the engine, fuel tanks etc) the CG will see a bit of a shift but will not be that drastic. The biggest weight shift will be moving the mains up front. However since your gear is located in a different place and thus is loaded differently your CG limits may change.

Not completely sure what you are asking. But in a tail dragger aircraft the center of gravity (CG) is behind the main land gear (MLG), while a tricycle configuration places the CG in front of the MLG. You achieve this by placement of the MLG, tail draggers generally have their MLG mounted near or forward of the lending edge of the wing, while tricycles' MLG are placed toward the middle or aft part of the wing.
Of course moving the landing gear around will move the empty CG of the aircraft and a tricycle configuration will generally weigh more.
There are many kit planes that allow you to build the aircraft either way, but I don't know any that would allow an easy conversion from one to the other.


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At what altitude did Concorde go supersonic?
At what point during flight would Concorde go supersonic? Would it climb to cruising altitude first? Does altitude affect the transsonic difficulties (buffet, need for extra thrust, etc.)?

Concorde went supersonic during its climb phase to the cruising altitude.
The altitude at which the Concorde can go supersonic was limited by the regulations. For example, the aircraft was only allowed to go at 400 knots during the climb phase between 6,000 and 32,000 ft; also, the aircraft was allowed to go supersonic only after it has crossed the coastline.
As the air gets thinner during climb, the Mach number increases for same IAS, reaching around 0.93 at 25,000 ft and 0.95 at 28,000 ft. After this, the afterburners are engaged and autopilot is set to the cruising altitude (~ 60,000 ft). During this climb phase to the cruising altitude, the aircraft goes supersonic, settling at around Mach 2 at over 50,000 ft.
The flight envelope limitations of Concorde (at one configuration) is shown in this image:

Source: pprune.org
The altitude affected the thrust requirements, as more thrust is required for the same speed at lower altitudes due to density. This is one of the reasons for Concorde flying at altitude far above that of subsonic airliners.

If it took off from Paris would it still be climbing at the coastline? Or would it hold at a lower altitude until it got there?

@TomMcW: Probably would depend on the precise trajectory it flew out of Paris - the coast is much closer going northwest than going west, and much closer going west than going southwest.

Also, I find it interesting that the Concorde's stickshaker behaviour is defined for altitudes exceeding 60 kilofeet, despite altitudes that high being well outside the Concorde's flight envelope.

@Sean looks like they went up to 68 during tests.

On the North American routes the Concorde out of Paris would mostly fly out in north westerly direction from Paris anyways due to being the great circle course. On the South American and Asian routes the Concorde would fly a lot longer over land and not go supersonic here due to regulations.